Offshore wells are often drilled from a stationary platform set in the water. Typically, several wells are drilled from the same platform because of the expense of the platforms and drilling operations and to facilitate servicing of completed wells.
Following completion of the wells, it is often necessary to do "workover" operations, such as replacing down-the-hole equipment, acidizing, fracturing, and wash-out operations. These operations are performed with a "workover" rig which is similar to a drilling rig, but usually smaller because it is not required to carry as heavy weights of pipe. "Workover" rigs may also be used for the drilling of relatively shallow wells. After installation on a platform, the workover rig must be moved over each well to service the wells drilled from the platform.
In the past, offshore platform drilling and workover rigs were moved to different locations on the offshore platform by pushing or pulling the rigs along greased metal-to-metal tracks. This method required a large amount of force to overcome the sliding friction between the rigs and the tracks. Subsequently, carriages consisting of "chain rollers," such as those manufactured by Hilman Rollers, 2604 Atlantic Avenue, Wall, N.J. 07719, were tried on rigs. The resistance encountered in moving the rig was reduced from a sliding friction to a rolling friction when these chain rollers were used; however, an external means of power to move the rig was still required. Typically, a block and tackle cable system or a system of hydraulic cylinders and skidding claws were used as the moving mechanism. (One example of hydraulic cylinders being used is disclosed in U.S. Pat. No. 3,802,137 to Armstrong). The block and tackle cable system was disadvantageous because it was tedious and slow, and also because it was impractical where only the upper structure of a rig, such as that disclosed in U.S. Pat. No. 3,802,137 to Armstrong, was to be moved. In the hydraulic cylinders and skidding claws system, the hydraulic cylinders provided the force to move the rig structure while the skidding claws provided an anchoring point. The mechanism had to be anchored before any force could be exerted and the rig structure could only be moved for a distance equal to the stroke length of the cylinders. The cylinders would then be contracted, the skidding claws moved to their next anchoring point, and the process repeated. This process was obviously repetitive, tedious, and slow.
It was therefore an important objective of these new inventions to provide a mechanism that combined a high load-carrying capacity with a means for mobilizing the load carried by the mechanism and that would not require that any external force be applied. It was also an important objective of these new inventions to provide a mechanism that would allow the load to remain stationary once it had been moved to the desired position.